Protective relay



April 20, 1965 c, A. MATHEws PROTECTIVE RELAY 4 Sheets-Sheet 1 Filed Aug. l. 1961 Charles A. Mathews, b5 wwf S. RWM AttovT-n eg.

April 20, 1965 c. A. MATHx-:ws

PROTECTIVE RELAY 4 Sheets-Sheet 2 Filed Aug. l, 1961 KNSM H Inventor: Charles A.Mathews, b5 @um 5. MNM

April 20, 1965 c. A. MATHEws PROTECTIVE RELAY 4 Sheets-Sheet 5 Filed Aug. 1. 1961 C/RCU/T CURRf/VT Inventor: Charles A. Mathews, b5 (LK/d S. Attorneg.

April 20, 1965 c. A. MATHEWs PROTECTIVE RELAY 4 Sheets-Sheet 4 Filed Aug. l, 1961 Inventor: Charles A. Mathews,

Attorneg.

United States Patent() "ice 3,179,850' PRTECTIVE RELAY Charles A. Mathews, Springfield, Pa., assigner to General Electric Company, a corporation of New York Filed Aug.l 1, 1961, Ser. No.128,472 9 Claims. (Cl. 317--3d) This invention relates to protective relays, and more particularly it relates to a relay which operates in delayed response to the occurrence `of abnormal conditions in an electric current circuit, with the delayed response being obtained by utilizing electric energy storing means including a reactance element such as a capacitor.

It is common practicein the art of protective relays Yto protect electric lines or circuits by means of relays designed to respond to abnormal circuit conditions with a time delay inversely related to the severity of the abnor- Hmality. For example, the '-overcurrent protective relay "tory, there is now a discernible trend in the relay art to accomplish the same functional result by means of staticfcircuitry, Vi.'e., by utilizing appropriate combinations of semiconductors and other solid-state components having n moving parts. In order to obtain the requisite time delay in the operation :of a static overcurrent relay, itis the usu practice to employ an electric energy storing circuit including a D.C. energized capacitance element and the like. Energization ofthe energy storing circuit is 'controlledby a D.C. signal derived from the protected ,cir-cuit, and the capacitance element serves to delay relay operation according to the value of that signal.

ln one such relay, which is fully described and claimed in Patent No. 3,079,533 granted on February 26, 1963, to W. C. Kotheimer and assigned to the assignee of the present inventiorn the timing capaci-tor of the energy storing circuit is supplied with 1a rapid succession of triangular D.C.v energizing pulses whose peak magnitude and duration are both dependent upon the value of circuit current,

f and the capacitor is charged in a series of rapidly recurring'sm-all increments. The relay operates after a delay ndetermined by the time required for the cumulative charge on the timing capacitor to attain a predetermined critical level. With this arrangement an approximation of a true -lzt operating characteristic is inherently obtained over some ofthe operating range of the relay. n

Gne objectv of my invention is to provide an improved and more sophisticated embodiment of theKotheimer relay.` n

A" general object of `the present invention is the pro- 'vision oan improvedinverse-time-overcurrent'relay of 4the kind utilizing `a timing capacitor the energizationV of Vwhich is controlled by a D.C. signal vwhose magnitude is y'dependent on the value of current in a protected circuit,

vthe relay being provided 'with sarturable transforming v fme'kansfor exerting a'limiting influence on the magnitude of theDfC. signal atthe higher? levels of overcurrenty V#within the e'X-pectable operatingran'geof the relay; i It has n: alreadyfbeen-mentioned* that relay operation 0c- 'curs' when the incre'mentally` accumulated charge on the vtiming capacitor attainsV a predetermined critical level., .The attainment of this critical level iis Adetected by suitable level detecting means connected to the timing capaci- 3,179,850 Patented Apr. 2li, 1965 tions of relatively slight severity, when the relay operating time is longest and the capacit-or charge nears its critical level at a very gradual rate, there may be a tendency for the relay to stall or fail to operate. This stalling problem, which is fully explained in the detailed description of my invention to follow, is particularly acute `Where a double-base diode or unijunction transistor is used to perform .the level detecting function. Therefore, another object of the present invention is the provision, in a protective relay of the kind utilizing a timing capacitor which is energized by a train of D.C. energizing pulses and having level detecting means responsive to the attainment of a predetermined critical charge by the capacitor, of means guaranteeing the reliable response of the level detectingmeans even under long-time operating conditions.

A further object of my invention is to provide an improved inverse-time ove'rcurrent relay incorporating a timing capacitor and Aa unijunction transistor level detector, the relay being capable of faithful and reliable operation :after a relatively long time delay under low-grade over-current conditions.

In carrying out my invention in one form, I provide condition responsive means adapted to be coupled to an alternating current circuit for deriving therefrom a D.C. signal which is dependent upon circuit current. An electric energy storing circuit, including a capacitor, is disposed for energization by an electric quantity which varies in accordance with the magnitude of the D.C. signal, and level detecting means connected to the capacitor is arranged to initiate a predetermined control function, such as initiating an opening operation of a circuit interrupter, when the capacitor is charged to a predetermined critical voltage level. It is characteristic of thiscircuitry that the capacitor voltage will attain its critical level, following an increase in circuitcurrent, after a time delay which is inversely related .to the vamount of current increase. In order to obtain desired short-time perform- 'an ce should the amount of current increase exceed a predetermined value, l utilize as part of the aforesaid condition responsive means, saturable transforming means designed to begin saturating, and to become progressively more saturated, in response to circuit current increasing beyond said value, the D.C. signal which is derived from the alternating current circuit being linearly representative of Y circuit .current `for cur-rent increasesV up to that value.

In another aspect of my invention, the above-mentioned level detecting means is arranged for nominal response to a level of capacitor voltage higher than said predetermined critical level, and periodically active sampling means is provided in association'with the level detecting means for effecting, at short time intervals, an increase in the sensitivity of the level detecting means soA that it is able to respond to the predetermined critical voltage level. This arrangement will preventy stalling of the relay 'when the amount of circuit current increase is small and the capacitor voltage is rising very gradually to its critical level.

My invention Will be better understoodV and its various objects f and advantages will be more fully appreciated from 'the following description taken in conjunction with the accompanying drawingsin which:

FIG. l is a schematic circuit diagram, partly inblock form, of anmelectric current circuit protected by 'a relaying system embodyingmy invention; FlG. 2 is aselie'matic circuitdiagram of the protective relay shown. in"block form in FIG. 1,the relay circuitry in thisgure illustrating apreferredvembodiment of my invention; v

- tor. fl have found thaty for lowagrade overcurrenl condi- FIG. 3 yis a graphot the;inVerse-time-overcurrent operating characteristic of the particular relay means shown in FIG .2; s f

FIGS. 4 and 5 arevoltage ys. time charts setforth to advance a simplified explanation of the operation of `the illustrated relay;

FIG. 6 is another voltage vs. time chart to facilitate a clear understanding of the mode of operation of the relay; and

FiG. 7 is a perspective view of representative transforming means which is a circuit component of the relay shown in FIG. 2 in accordance with a preferred embodiment of my invention.

Referring now to FIG. 1, there is shown in block form a protective relay system for a B-phase alternating current electric power circuit. The illustrated circuit comprises three wires 1a, 1b and 1c which are used, for instance, to interconnect to a 3-phase source of power and appropriate load apparatus, neither of which are shown. The connection between circuit and power source is controlled by means of a conventional 3-pole circuit breaker 2, shown closed.

In order to provide 3-phase protection for the circuit Melli-1c, identical single-phase relay means 3a, 3b and 3c are providedrespectively, for the three different circuit phases, and for the sake oi. drawing simplicity the contents of only relay means 3a have been indicated in FIG. 1. Each of the relay means is designed in a like manner to initiate an opening operation of the circuit breaker 2, thereby disconnecting the protected circuit from its power source, in response to the occurrence of an abnormal circuit condition involving that phase with which the responding relay is associated.

Relay means 3a, 3b and 3c are each arranged to be energized in accordance with a characteristic electric quantity of the associated circuit, and relay response is intended when this quantity, as a result of an abnormal circuit condition, attains a predetermined pickup value. In the illustrated application of the relaying system the characteristic electric quantity is alternating current, and abnormal conditions, such as overloads or short circuits, are indicated by the circuit current rising above its normal, full-load value-the degree of overcurrent being dependent upon the severity of the abnormal condition. In order to obtain this overcurrent response, three star-connected instrument current transformers lia, 4b and 4c are respectively coupled to the wires 11a, 1b and 1c of the protected circuit, and as can be seen in FIG. 1, the secondaries of these current transformers are connected to the relay means 3a, 3b and 3c, respectively.

In the relay means 3a, the secondary current from the current transformer da supplies input to both a time delay channel and an instantaneous channel of components. The instantaneous channel, which is designed to respond with no intentional time delay, comprises the following chain of functional components: impedance means 5, rectifier 6, instantaneous pickup adjustment means 7, and a level detector S. The impedance means 5 derives an AfC. voltage representative of the current ilowing in circuit la, and this is converted to a D.C. quantity by the rectiiier 6. The components 7 and S comprise suitable means for substantially instantaneously producing an output control signal at 9a when the D.C. quantity supplied by rectifier o reaches a pickup magnitude corresponding to a predetermined amount o overcurrent in the circuit 1a. Component 7 is preferably adjusted so that this predetermined amount of overcurrent is a high multiple of normal current, because instantaneous operation of the relay means 3a is usually desired only for the most severe abnormal or fault conditions.

The output control signal at 9aof the relay 3a activates a high-speed static switch 10 which, by way of example, may comprise a silicon controlled rectifier., As is indicated in FIG. 1, the static switch 1t) will also be Vactivated by output control signals produced-at 9b and 9c upon operation of the instantaneous channelsinthe companion relay means 3b and 3c respectively. 1n lieu of astatic switch, component 16 could comprise an electromagnetic relay or some other circuit controlling device if desired. When activated the component 1t) is instantly effective to complete a tripping circuit for the circuit breaker 2. The tripping circuit includes in series relation a battery 11, a normally open auxiliary contact 12 of the breaker 2, and a trip coil 13. When energized by the battery 11, upon activation of the static switch 10, the trip coil 13 actuates a latch 14 thereby releasing the switch member of the circuit breaker 2 for rapid circuit opening movement.

The battery 1'1 is also used to provide control power as needed for the various relaying circuits. For this purpose a Zener diode 15, or some other suitable voltage regulating device, is connected in series with a voltage dropping resistor 16 between the battery terminals, as shown. A smoothing capacitor 1'7 is connected across the Zener diode 15 to aid in absorbing unwanted voltage surges. A Zener diode having a voltage breakdown level of about 20 volts is preferably used, and hence this device comprises a 20-volt source of regulated D.C. supply voltage. The positive and negative D.C. supply voltage terminals are identified throughout the drawings by encircled plus and minus symbols, and respectively.

The remaining components in the relay means 3a comprise the various parts of a time delay channel which is designed to develop another output control signal at 18a in delayed response to the occurrence of an overcurrent condition in the circuit 1a, with the amount of delay being inversely related to the severity of the condition. In other words, this channel is to operate with a delay which is longer at small overcurrent values than at higher overcurrents. As is indicated in FIG. 1, the output control signal at 18a is supplied to a second high-speed static switch 19 which, like the previously mentioned switch 1t?, completes the tripping circuit for the circuit breaker 2 when activated. The static switch 19 can also be activated by output control signals produced at 18b and 18e upon operation of the time delay channels in the companion relay means 3b and 3c, respectively.

The time delay channel in the relay means 3a includes suitable non-linear impedance means 20 which is energized by the secondary current of the current transformer 4a. This non-linear impedance means 20 in combination with a rectilier 21 comprises condition responsive means for deriving from `the protected circuit 1a a C. signal the effective magnitude of which is dependent upon the value of circuit current. The reason for using non-linear impedance means will be explained in the more detailed description of my invention set forth hereinafter.

As is shown in FIG. 1, the D.C. signal provided by the rectifier 21 of the condition responsive means is added in an adding circuit 22 to a pulsating triangular signal which is supplied by a sawtooth generator Z3 or the like. This pulsating signal comprises a succession of triangular-waveform signal pulses of substantially constant peak magnitude, and the inherent rate of recurrents or frequency is relatively high compared to the frequency ot' the current in circuit 1a.

The adding circuit 22 provides a resultant signalcorre- 'sponding to the sum of the D.C. signal and the pulsating triangular-waveform signal, and the resultant signal is passed through a bias component 24 which subtracts therefrom a unipolarity reference signal level equivalent to the peak magnitude of the pulsating signal. Thus the output of the Abias component 24 comprises a rapid succession of triangular D.C. pulses, both the amplitude and the duration of every pulse being directly proportional to the magnitude of the D.C. signal and hence being dependent upon the current valueV in the protected circuit 1a.

An adjustable timing circuit 25 is next provided for energization in accordance with the output of component 'bolically in FG. l.

lstantially instantaneouslyto produce the aforesaid outponent 27 is coupled to the protected circuit lla by meansy of the non-linear impedance meansl 20, a rectifier 28 andA a suitable level detector 29. The function of this chain of components is to keep the timing circuitZi` inactive or disableduntil an abnormal circuit condition occurs, as evidenced by the rise of circuit current above normal.

' The value of circuit current to which the starting and resetting control responds Vis designated pickup current. Once pickup current is attained, the timing circuit 25 is able immediately to start its timing function. Should the circuit current return to lnormal before the timing operation is finished, the starting and reset control 27 is effective to quickly and fully deenergize or reset the reactance element in the .timing circuit, thereby avoiding the possibility that residual energy accumulation in the reactance element might undesirably shorten the relay operating` time yif another over-current condition were to occur soon thereafter.

Due to the above-mentioned form of energization supplied to the adjustable timing circuit 25 of the relay means 3a, the time required for its reactance element to accumulate the aforesaid critical energy level, following the' occurrence of an overcurrent condition, is inversely proportional to an exponential function (approximately the square) of the D.-C. signal magnitude, and therefore Vthe time delay channel of components liti-29 of the relay means shown invFlG. 1 operates with the desired inversetime-overcurrent characteristic. The operating characteristic actually obtained has been graphically illustrated in FIG. 3 which is a conventional time vs. current graph. In FIG. 3, both `coordinates are scaled logarithmically, andthe amount of current in the protected circuit, in terms of multiples of pickup, is plotted along the a-bscissa.

The curved line 30 in FIG. 3 defines the relay operating characteristic for one particular time adjustment, and its approximation of a true inverse-square current-time relationship is apparent by comparing curve 30` to the straight line labeled igt. In applying the present relay, its operating characteristic commonly is required to be selectively coordinated'with the inherent operating characteristics of other protective devices, such -as electric fuses or electromechanical relays, and for this reason the low-overcurrent (less than about three times pickup) end or" the operating characteristic 3d has, as will be observed in FiG; 3, been made to depart in an extended time direction from the lz line. The manner in which this desired departure is obtained will be explained in the detailed description ofthe relay .to follow. For the same reason, at `high multiples of pickup which accompany severe overcurrent conditions, relay operation has been prolonged relative to a true inverse-square responsive time, and this is illustra-ted in FlG. 3 where the relay operating characteristicV 3@ will be seen to deviate, in a longer-time sense, from the Pt line at its high-overcurrent (more than about six times pickup) end. The manner of obtaining this; desired deviation will be explained in the detailed description which follows. With reference novvV to FIG. 2, a detailed circuit description will be given of the preferred embodiment of the time delay channel of the relay means 3a shown sym- The illustrated relay is adapted to be inductively coupled, by means of the current transformer du, to the alternating current circuit which is represented in FIG. 2 at lla. The secondary Winding of the current transformer is connected to saturable transforming means 31 which comprises the non-linear-impedancemeans component 2@ of FIG. l. The transforming means 3l includes primary and secondary windings 31a and Sib, respectively, `and a magnetizable core 31C having an air gap. The primary Winding Sila, which is connected to the current transformer 4a, is provided with a plurality of preselected taps 32a and 32h so that the number of turns energized by the secondary current of the currenttran-sformer can be conveniently changed. In this Way the pickup setting of the relay, measured in terms of current transformer secondary amperes, can be changed to suit the particular needs of several dilerent relay applications.

The transforming means 3l derives across its secondary winding Sib an A.C. voltage which is dependent upon the alternating current in the circuit la. This transforming means is designed, in a manner to be more fully described hereinafter, so that its secondary voltage is linearly representative of the circuit current for a predetermined first range of overcurrent values. When current values exceed this first range, however, saturation begins and the secondary voltage increments Will become progressively smaller as the secondary voltage approaches a predetermined maximum level. It is because of this non-linear relationship in the higher overcurrent region that the desired deviation of the relay operating characteristic from an Izt line at high multiples of pickup (see FIG. 3) is obtained.

Rectifying means, preferably comprising the fullWaVe bridge type rectifier 33 illustrated in FIG. 2, is connected to the secondary Winding 31b of the transforming means 31 in order to provide the D.C. signal which is used to control the energization of the timing circuit of the relay. The D.C. signal comprises a succession of unipolarity half-cycle waves representative of the A.-C. voltage supplied to the rectifier, and hence representative of the alternating current inthe circuit 1a; no filtering or smoothing means need be used. It is apparent, therefore, that the rectifier 33 provides a D.C. signal (voltage) the effective magnitude of which is dependent upon the. value of the circuit current. Since the transforming. means 31 is in its linear region (not saturated) during relatively mild overcurrent conditions, the effective magnitude of the D.C. signal will be directly proportional to the amount of circuit current throughout the aforementioned first range of overcurrent values.A

The negative and positive D.-C. terminals of the rectifier 33 are connected, respectively, to conductors A andv B. Conductor A is connected to the negative supply voltage terminal (the encircled minus symbol) through a surge suppressing capacitor 34 of very small capacitance. Conductor B is connected to a resistor T15 which is supplied With a pulsating signal of triangular Waveform' by a pulsating signal (voltage.) source comprising, in the pre.- ferred embodiment of the relay, the, sawtooth generator 23. The sawtooth generator circuitry has not been shovvn in detail since I contemplate that it will be conventional. As a practical example, circuitry such as that described and claimed in United States Patent No. 2,792,499-Mathis, granted on May 14, 1957, could be used.

The output circuit of the sawtooth generator 23 is coupled by means of a capacitor 36 to the first winding 37a of a step-up transformer 37, and there is consequently developed in the second transformer winding 37b an A.C. signal having a triangular Waveform of substantially constant amplitude. The parameters of the generator 23 and the turns ratio of transformer 37 are selected so that the amplitude (peak magnitude) of this signal is a predetermined desired height (for example, 2O volts),

and the signal frequency is arranged to be relatively high for example, 2,000 cycles per second) compared to that of the alternating current in circuit 1a (6() cycles per second, Vfor example). The transformerwinding 37b is connected across the resistor 35 vvhich, inturn, is connected between the conductor B and a conductor C. As is apparent in FIG. 2, the conductors A, B and C comprise adding means providing, between conductors A and C, -a resultant signal corresponding to the sum of the D.-C. and A.C. signals appearing at rectifier 33 and transformer winding 37b, respectively. This is the adding circuit 22 of FIG. 1. The potential of conductor C, measured with respect to conductor A, is equal to the instantaneous magnitude of the D.-C. voltage across the rectifier 33 plus (or minus, during negative half cycles) the instantaneous magnitude of the serially related A.-C. voltage across the resistor 35. This adding function could be accomplished in ways other than by the series circuit shown; for example, the D.C. signal and pulsating triangular signal might comprise different currents flowing through a common resistor, whereby the resultant voltage drop across the resistor would reliect the sum of the superimposed currents.

In association with the adding means A, B, C illustrated in FIG. 2, appropriate impedance means 41 is provided for introducing, in subtractive relationship with the resultant signal which is developed between conductors A and C, a unipolarity reference signal level equal to the peak magnitude of the pulsating triangular-waveform signal appearing between conductors B and C. The impedance means 4l thus performs a biasing function, and it is the circuit element of the relay corresponding to the bias component 24 shown in block form in FIG. l. Preferably, as is shown in FIG. 2, the element 4l is a Zener diode or the like, such a device having an abrupt breakdown characteristic which enables it to block the iiow of reverse current as long as the reversely poled applied voltage is less than a predetermined breakdown level, at which point it enables the reverse current to flow while limiting the voltage drop thereacross to the predetermined breakdown level.

The voltage breakdown level of the impedance means 4l is the unipolarity reference signal level referred to. In the illustrated embodiment of the relay a Zener diode 4l having a 20-volt breakdown characteristic is used, and this determines the desired height of the A.C. voltage amplitude across winding 37b. Hence the reference signal level is equivalent to the peak magnitude of the triangular-waveform pulsating signal supplied by the sawtooth generator 23. It will be understood by those skilled in the art that circuit elements other than a Zener diode might be satisfactorily used for establishing the unipolarity reference signal level; for example, a resistor energized by an appropriately poled 20-volt bias battery would have a similar effect.

As can be seen in FIG. 2, the Zener diode 4l. is serially connected between conductor C of the adding circuit and another conductor D, and it is poled in opposition to the polarity of the rectifier 33. Accordingly, its breakdown voltage is subtracted from the resultant voltage provided across conductors A and C, and the dierence or net voltage taken across conductors A and D is supplied to a resistor 42 connected therebetween. In order to ensure that the potential of conductor D will never go negative with respect to conductor A, an appropriately poled diode 43 is connected in parallel with the resistor 42, and another diode 44 is serially inserted between conductor C and the Zener diode 4l, as shown.

The various voltage relationships described thus far have been graphically illustrated in FIG. 4 for two cycles of the sawtooth generator 23. In FIG. 4 the D.-C. signal provided by the rectifier 33v (the potential VDJ, of conductor B taken with respect to conductor A) is represented by the height of the dot-dash line b above the base line a, and for the sake of a simplified explanation it is shown at a constant magnitude of about ten volts. The regular succession of triangular-waveform signal pulses supplied by the sawtooth generator 23 (the voltage between conductors C and B, having a substantially constant amplitude Vs), is represented by the trace c which is symmetrical with respect to the line b. From the resultant voltage measured between conductors A and C of the adding circuit (between the base line a and trace c in FIG. 4), the Zener diode reference voltage level VZ is subtracted. When and to the extent that the resultant voltage magnitude exceeds the reference voltage level, a net voltage is developed across resistor 42 (between conductors A and D), and this is represented in FIG. 4 by the heavy line d.

It will be observed in FIG. 4 that the net voltage d comprises a succession of triangular D.C. pulses whose frequency and waveform correspond to those of the pulsating triangular signal C, and the maximum or peak magnitude of this voltage has been identified as Vn. Since VZ and Vs are selected to be equal to each other, Vn is necessarily equal to VD, C,. As a result, both height and base of each net voltage pulse are directly proportional to the magnitude of the D.-C. signal derived from the alternating current circuit la, and the area of ,each pulse is directly proportional to the square of that magnitude. The diodes 43 and 44 shown in FIG. 2 aid in clipping negative-going portions of the resultant voltage-whenever the conductor C is negative with respect to conductor A, as indicated by the broken-line portions of trace c in FIG. 4, the potential difference between conductors C and D will be impressed across the diode 44.

In FIG. 2 it will be seen that the adding circuit A, B, C, with the impedance means 4l in series therewith, has connected thereto electric energy storing means comprising the series combination of resistance elements 4S and 46 anda normally deenergized reactance element 47. This energy storing means is the adjustable-timing-circuit component 25 of FIG. 1. As shown in FIG. 2 the reactance element 4'7 is a capacitor, and preferably one having a capacitance of six microfarads is used. The resistance elements 45 and 46 are both potentiometers which enable time adjustments to be made in the operating characteristic of the relay means 3a. The potentiometer 46 has a relatively large total resistance, such as 500,000 ohms, and itis provided with a plurality of taps at predetermined intervals for eld selection of the desired time setting. The Vernier potentiometer 45 is used for precise factory adjustment.

The elements i5-47 form an RC circuit, and this circuit is connected across the resistor 42 for energization in accordance with the net voltage developed between conductors A and D. As is shown in FIG. 2, a diode 48 is disposed between the potentiometer 45 and conductor D to block discharge of the capacitor 47 into its D.C. energizing circuit. The relay circuitry hereinbefore described causes periodic energization of the RC circuit by a train of triangular-waveform voltage pulses recurring at a relatively high frequency (2,000 c.p.s.), and the timing capacitor 47 will be charged in a rapid succession of small steps or increments by this pulsating energizing quantity. The maximum magnitude and duration of each energizing pulse varies in accordance with the magnitude of the D.C. signal (at rectifier 33) which is derived from the alternating-current circuit 1a.

The capacitor 47 is normally maintained in a discharged state by supervising means 49 associated therewith, and a description of the supervising means will follow hereinafter. Before that, however, it is appropriate to note that when capacitor charging is permitted, the time required for it to accumulate a predetermined amount of energy will be inversely proportional to approximately the square of the maximum magnitude VI, of the energizing quantity. This will be best understood referring to the greatly simplified example illustrated in FIG. 5.

FIG. 5 is a chart of capacitor voltage vs. time for two complete cycles of the sawtooth generator 23, and two different circuit conditions have been illustrated. In the rst instance, the maximum instantaneous magnitude of the train of triangular energizing pulses d1 supplied to the RC energy storing means is V111. Assuming that capacismesso tor charging is permitted as of time O, the resulting voltage buildup across the capacitor 47 as energy is accumulated therein is represented by the solid line h. in actual practice, a period of the pulsating energizing quantity d1 is much shorter than (less than one-iiftieth, for example) the time constant of the series RC energy storing circuit, rand consequently the energy increment in capacitor 47,

Vper energizing pulse, will be directly proportional to the product of the pulses maximum magnitude and the time during which it effects charging of the capacitor. After two cycles (at time T in FIG. 5), the capacitor 47 has been charged to a voltage level Vpn., and for the sake of `illustration it will be assumed that at this point the energy accumulation in the capacitor is just equal to the aforesaid predetermined amount.

For the second condition shown in FIG. 5, it is assumed that there is a greater amount of overcurrent in circuit la so that the maximum magnitude of the D.-C. energizing pulses (broken line d2) has increased to \2 Vm. Under this new condition, voltage-builds up across capacitor 47 in the manner represented by the broken line 51 in FIG. 5. The area of each of the larger triangular lpulses d2 is twice that of each of the original pulses d1, and the capacitor charge at the end of the initial cycle of the new condition is double what it had been in the nating current circuit.

An exact inverse-square relationship is not obtained -due tothe effect which prior energy accumulation in the capacitor 47 has .on each subsequent 'period of capacitor charging. It is apparent from a consideration of FIG.

5 that successive periods of capacitor charging will become progressively shorter as the level of capacitor volt- .age rises, and the charge increment per cycle of the sawtooth generator decreases with time. Thus the critical Voltagelevel VDM, which has been used as an example is slightly under twice the value of the capacitor voltage Sii at the end :of the irst sawtooth generator cycle, and this .level is attainedvby the broken-line voltage 51 just prior to the time 1A. T.

A more eXact'inverse-square relationship can be ob- .tained,kif desired, by inserting between the impedance means 41 and the resistor 42 of FIG.` 2 a low-pass filter (not shown) arranged to der-ive a continuous D.C. quan- Vtity the average magnitude of which is directly proportional Vto the square of the magnitude of the D.C. signal at rectier 33; In accordance with this alternative, the energy storing means of theV relay in effect comprises two separate sections:A a ,first section `(the low-pass filter) rwhich squares the difference or net voltage developed between conductors A and D; and a second section (the RC timing circuit) which integrates the squared quantity. But Ik prefer the relay embodiment shown in FIG. 2 becauseiit yields a satisfactory degree of inverseness without the Iadded expense and somewhat prolonged operating time (at high multiples of pickup current) which would attend theuse of `a low-pass filter.

Returning to `FlG.v 2, the supervising means 49 which normallyl prevents the accumulation of energy in the .capacitorV 47 will now be described. This supervising means lcomprises two impedance elements or resistors 53 and 54 connected from the positive and negative D.C. supply voltage terminals, respectively, to differently poled terminals of capacitor 47, and a diode 55V connected between these two resistors in parallel relationship with vthe capacitor.V `The positive electrode or anode of the diode 55 is connected to resistor 53; consequently it is poled in a blocking disposition relative to the D.C. energizing circuit of capacitor 47 while being normally conductive with regard to current flowing from the positive supply voltage terminalthrough resistors 53 and 54 to the negative supply voltage terminal. A third resistor '56 is connected directly between the resistor 53 and the negative supply voltage terminal, across the combination of diode 55 and resistor 54. A second diode 57, poled in a conducting disposition relative to the capacitor energizing circuit, is serially connected between the negative electrode or cathode of the diode 55 and the relatively positive terminal of capacitor 47. It is apparent that while the diode 55 is conducting (forward biased), the potential difference across the terminals4 of capacitor 47 must necessarily be negligible, and no appreciable energy can be accumulated or stored therein.

The supervising means 49 includes normally inactive shunt circuit means 58 connected across the combination of the resistor 53 and the diode 55 in series therewith. The shunt circuit means, which preferably comprises the emitter-collector circuit of a PNP transistor 59 as is shown in FIG. 2, is arranged to provide, when active, a relatively low-impedance path between the positive supply voltage terminal and the cathode of diode 55. Consequently, activation of the shunt circuit means 58 renders the diode 5S non-conductive (reverse biased) and hence enables charging of capacitor 47 to take place. The second diode 57 keeps positive supply voltage potential from appearing on the relatively positive tenminal of capacitor 47 when the shunt circuit means 58 is in its active (low-impedance) state. Whenever it is desired to have the normally deenergized capacitor 47 begin accumulating energy, the shunt circuit means 58 is activated by suitable starting means 60 associated with supervising means 49. p

The supervising means which has been described hereinabove comprises the starting-and-reset-control component 27 of FIG. l, and it is the claimed subject matter of a copending patent application, S.N. 128,421 led on August l, 1961, for C. A. Mathews and E. M. Smith and assigned to the assignee of the present invention.

The starting means 6i) illustrated in FIG. 2 comprises Va full-wave bridge type rectifier 61 (the rectifier component 2S of FIG. 1) and the level detector 29 (also shown in FIG; 1). The output circuit 62 of the level detector is connected to the base electrode of the transistor 59 which is part of the supervising means 49. The input` signal for the level detector 29 is provided by the rectifier 61 Whose A.C. terminals are connected through a resistor 63 land a 1:1 ratioisolating transformer 64 to the secondary winding 31h of` the transforming means 31. Thus the level detector input and the D.-C.`signa1 provided by rectifier 33" for the adding circuit 22 described hereinbefore are both derived from the same source and have equal magnitudes, the magnitudes of both being correspondingly dependent upon the value of alternating current in the circuit 1a.

The level detector 29 comprises any suitable circuit arrangement capable of producing a negative-going output signal in high speed, switch-like response to its input signal attaining a predetermined pickup level. Since it ,is thought unnecessary to show circuitry details for a complete understanding of the present invention, reference is made to Patent No. 3,067,340 granted on December 4, 1962, to M. E. Hodges and assigned to the assignee of the present invention, in which a level detector particularly well suited for the present purposes is fully described and claimed.

The output circuit 62 of the level detector 29 is cou- `pled to the positive supply voltage terminal by a load resistor 65, and the emitter-base junction of the transistor 59 is connected across resistor 65 as is shown in FIG. 2 When the level detector input signal reaches its` pickup level, the immediately resulting output signalcauses a voltage drop across the load resistor 65 and current flow is effected in the emitter-base junction of the transistor 59. This forward bias of the emitter-base junction activates or turns on the transistor, and consequently the shunt circuit means Se is changed from its normally inactive (high impedance) state to an active (low impedance) one. As a result, the normally conducting diode S of the supervising means 49 is rendered non-conductive, and the capacitor 47 can start charging.

In a preferred embodiment of the invention, the predetermined pickup level to which the level detector 29 responds is selected so that the starting means 6i) will operate as described above when the value of the voltage across the transformer secondary winding 31]) attains four volts R.M.S. This 4-volt value of secondary voltage corresponds to the pickup value of alternating current in circuit lla, the absolute value of circuit current at pickup being determined by the particular primary winding tap (32a, 3217) in use.

The level detector 29 is arranged to discontinue its output signal whenever its input signal falls below a predetermined dropout level, the dropout level preferably being selected to be at least 90 percent of the above-mentioned predetermined pickup level. Consequently, as soon as current in the circuit In decreases to a corresponding dropout value, the starting means 6i) (which preferably drops out with no intentional time delay) can no longer sustain a forward bias at the emitter-base junction of the transistor 59. As a result, the transistor 59 is turned off (rendered non-conductive), and the shunt circuit means 58 returns to its normally inactive state. This removes the reverse bias of diode 55, and whatever charge may have been accumulated by the timing capacitor 47 is quickly dissipated by the supervising means 49, thereby resetting the relay means 3a. The parameters of the supervising means 49 preferably are so selected that in less than one-sixth of a second, following the inactivation of shunt circuit means 5S, capacitor 47 is almost completely discharged or reset.

During normal circuit conditions, the shunt circuit means 58 of the supervising means i9 will remain inactive, and consequently the capacitor IS7 of the RC timing circ-uit is normally discharged and has no energy stored therein. However, in response to the occurrence of an abnormal circuit condition, as evidenced by the current in circuit la attaining at least its pickup value, the starting means 60 operates to activate the shunt circuit means 5S, and the capacitor 47 is able to begin accumulating energy from its D.C. energizing circuit as previously explained. Capacitor charging continues until the above-mentioned predetermined amount of energy has been accumulated, the time required for such accumulation being inversely related to the severity of the abnormal condition as reflected by the amount of overcurrent in the circuit In. At this point the capacitor voltage has built up to a predetermined critical level which is illustrated, by way of example, as Vp-u in FIG. 5. In order to ensure ample energization to cause ultimate relay operation whenever capacitor charging is permitted, even if the circuit current should only slightly exceed its pickup value, a critical voltage level Vpn, is chosen that is equal to the maximum instantaneous value of the D.C. energizing signal (the net or difference voltage between conductors A and D in FIG. 2) produced when circuit current has a predetermined value well below the aforesaid pickup value. Preferably this last-mentioned predetermined value of circuit current is less than one-half the pickup value, and the ytransistor and its unique operating characteristics are disl2 closed, for example, in United States Patent No. 2,769,- 926-Lesk, granted on November 6, 1956.)

Base-one of the unijunction transistor 67 (the lower base electrode as viewed in FIG. 2) is connected to the negative supply voltage terminal by way of a resistor 63, and base-two is connected to the positive supply voltage terminal by way of a resistor 69. For improved temperature stability, the resistances of resistors 63 and 69 preferably are selected to be equal to each other. The emitter of the unijunction transistor 67 is connected directly to the relatively positive terminal of the timing capacitor 47.

So long as the potential of the unijunction transistor emitter is less positive with respect to base-one than a characteristic peak point emitter voltage, the unijunction transistor 67 is cut off or inactive (and consequently its interbase impedance is high), and only quiescent current flows through the base-one resistor 63. When its emitter potential is raised to this critical peak point emitter voltage, however, the unijunction transistor 67 abruptly changes to an active, relatively low-impedance state and there is an appreciable increase in current owing through resistor 68. The succeeding relay circuits are designed to respond to this current increase, and hence it is the activation or ring of the unijunction transistor 67 of the level detector Z6 that initiates the output control signal of the inverse-time-overcurrent relay means 3a.

The relay circuitry thus far described, in its broad aspects, is the subject matter of the previously mentioned Patent 3,079,533-Kotheimer.

The particular unijunction transistor 67 which I prefer to use as the level detector 26 is arranged for activation when the peak point emitter voltage has a value of ten volts. Since this device is to be activated in response to the timing capacitor 47 being charged to a critical voltage level of only two volts, as has been explained hereinabove, an 8-volt bias is provided in series with the capacitor voltage. This bias is conveniently provided by the resistor 56, connected as is shown in FIG. 2 between the relatively negative terminal of capacitor 47 and the negative supply voltage terminal, in conjunction with the resistor 53 of the supervising means 49. By appropriately choosing the resistance values of the resistors 53 and 56, the voltage drop across resistor 56, with the diode 55 reverse biased, is made just equal to eight volts. Selection of the desired bias voltage can be facilitated by adding a potentiometer (not shown) in between the two resistors 53 and 56. As is apparent in FIG. 2, the emitter voltage of the unijunction transistor 67 before activation thereof comprises the voltage of the capacitor 47 plus the bias voltage across resistor 56.

The unijunction transistor 67 is activated or tired when the energy being accumulated in capacitor 47 attains its predetermined critical level, lwhereupon the capacitor 47 is quickly discharged through a path including the then low-impedance emitter-base-one junction of the unijuntion transistor and the resistor 68. Capacitor charging is accomplished in small, high frequency steps, as explained hereinbefore. The 'charging circuit includes the resistance elements and 46,*and these elements, in series combination with the capacitor 47, determine the time constant of the RC timing circuit. In order to obtain the desired maximum time delay in a reliable relay 3a of the smallest possible physical size, the capacitance of capacitor 4:7 has been kept relatively small and thel total resistance of elements 45 and 46 has been made quite large. This high resistance, which is in the emitter circuit of the unijunction transistor 67, could result in a stalling problem (to be explained below), particularly during low-grade overcurrent conditions when the emitter voltage of the unijunction transistor will be found approaching its peak point relatively gradually as the charge on capacitor'47 slowly nears its critical 2-volt Vlevel three seconds or longer after the start of capacitor charging.

A certain minimum value of emitter current (the peak i3 point current, which may be .000005 ampere, for example, at an interbase voltage or 20 volts) is required to tire a unijunction transistor, and the emitter circuit must be capable, of course, of delivering current of at least this value in order to activate the device. It is known that the rate of emitter current increase toward this peak point, immediately before firing, characteristically begins increasing when the emitter voltage applied to the unijunction transistor is less than one-tenth of a volt below its peak point. If the'rnagnitude of the quantity energizing the timing circuit of the illustrated relay were not appreciably in excess of pickup (tour volts), there is a possibility that the large resistance elements 44 and 45 might limit emitter current to a value less than peak point, in which case the peak point emitter voltage could not be reached. This possibility would materialize it the emitter current in the unijunction transistor 67, before activation thereof, were suticient to cause all of ther relatively small increment of charge accumulated by capacitor 47 during each triangular pulse just prior to the attainment of the critical firing level to be drained off (discharged) .beforethe next succeeding pulse appears. In other words, the peakpoint wouldV never be reached and the unijunction transistor 67 would stall if the last small increments of-capacitor charge necessary to raise the emitter voltage to its peak point were decrementally dissipated as rapidly as supplied.

To avoid the above-mentioned possibility of stalling, the unijunction transistor v67 which I actually use has a peak point emitter voltage whose nominal value is about twelvevolts, and periodically active sampling means 70 is associated therewith for increasing the sensitivity thereof to the above-mentioned l0-v01t level at predetermined time intervals. As can be seen in FIG. 2, the preferred sampling means 70 comprises another unijunction transistor 71 the base-one of which is connected directly to the negative supply voltage terminal. A resistor 72 is connected between the positive supply voltage terminal and base-two of the unijunction transistor 7,1, and the emitter of this device is connected through a current limiting resistor 73 to the junction of a resistor 74 and a capacitor 75 which are serially connected between the supply voltage terminals. Thus a well-known relaxation oscillator is formed, and it is coupled to the unijunction transistor 67 by means of a suitably small capacitor 76 which interconnects the base-two electrodes of both unijunction transistors 67 and 71. The transistor 71 of the periodically active sampling means is turned on for brief moments at recurrent intervals of relatively short duration.

While the sampling means 70 could be arranged if desired to operate at the frequency of the pulsating signal which energizes the timing capacitor 47, its parameters :preferably are so selected that .the predetermined time Aintervals between active'moments thereof are each from .two to eight times longer than the period of each triangular-waveforrn signal pulse supplied by the sawtooth .generator 23. `l In theillustrated embodiment of my invention the sampling means is arranged to operate at a frequency of 300 cycles per-second, and therefore there are 62/3 cycles of the sawtoothgenerator per cycle of the sampling means. Each time the sampling means70 is active, it .causes the 'base-two potential of the unijunction transistor 67k to be depressed by a sufficient amount to reduce'the peak point emitter voltage (which `is dependent upon the interblasevoltage) Lto .the Irequisite tenvolts.

Tl'ireehundred tinies `a second the periodically active sampling'fmeains '70 enables the unijunction transistor .67 of `the level-,detector 26 to respond if the capacitor 47 `has y`accuniulatedienough energy Vduring 4the preceding -`62g`energizing pulses to raise the total capacitor charge tonthe predetermined critical tiring level.` lt will be appaientthatrby using this arrangement I have virtually of the sampling means 70, the emitter voltage applied to unijunction transistor 67 will be well below its nominal peak point (above twelve volts), and the emitter current is then negligible. As a result, the amount of capacitor charge that can be drained off between consecutive energizing pulses is so very small that a significant net increase in emitter voltage is always obtained during each time interval, even if the interval overlaps a moment at which the half-cycle wave of the D.C. signal at rectier 33 goes to zero. At the expiration of an appropriate time delay, the emitter voltage will be increased during one of the sampling intervals from a value below to a Value at least las great .as the lO-Volt peak point, and when .the sampling means 70 is next active, firing of the unijunction transistor is assured. This beneficial result has been :achieved at the expense of :delaying the relay operating time by a maximum of three milliseconds which is not long enough, even under severe oveicurrent conditions when very-short-time response is desired, to affect adversely the performance of the relay.

It has been pointed out hereinbefore that when the unijunetion transistor `67.`iresA or is activated, there is an appreciable current increase in the base-one resistor 58. As is shown in FIG. 2, `the resistor 68 has connected in parallel circuit relationship therewith the emitter-base junction of a signal `amplifying NPN transistor 78. A current limiting'resistor 79 is connected in series with the base electrode of transistor 78, and a pair of silicon dio-des '80a and 80h, poled in agreement with the emitterbase junction, are serially connected to its emitter. The collector of this transistor is connected by way of a relatively small isolating resistor 81 and conductor 18a to an input terminal 82a ofthe stat-ic switch 19, terminal 82a being connected through `a load resistor 83 and another resistor 84 to the positive terminal of the supply voltage source. It is apparent, therefore, that the signal `amplifying transistor 78 will be turned on (become active) when its emitter-base junction is ttorward biased as a result of base-one current owing from the unijunction transistor 67 upon activation thereof.

The diodes 30a and 80b are' provided to ensure that 'the transistor 78 is not operated by the quiescent current of the unijunction transistor 67. Since each silicon diode inherently presents a relatively high impedance to the passage of a small quantity of `forward current, the quiescent current of the unijuiiction transistor 67 prefers to follow :a rpath through resistor 68, thereby avoiding activationof the transistor 78 which would take place it it were able tofollow the parallel path through the emitteribase junction of this transistor. As a result, the transistor 73 will remain inactiveuntil the unijunction transistor 67 has actually been fired.

The signal amplifying transistor 78, when active, produces a negative-going output control signal at the conductor 18a which emanates from the relay ine-ans 3a.

` tected circuit 1a ffrom its source of .power and hence ineliminated the .above-discussed problem of stalling.V Dur- Y ing theLOOS-second time intervalbetween active moments terrupting the overcurrent owing therein. Identical relay means (riot shown) associated with the electric current circuits 1b and 1c operate ,in the same way to supply similar output control signals Vto two other-input terminals S22-b and tile, respectively, .of the static switch 19, and all three input terminais are joined together as is indicated in FIG. 2.

The static switch 19- in the illustrated embodiment `of the inverse-time-overcurrent relay Vpreferably Vcomprises a solid state controlled rectifier 85. Asis shown in FIG.

the trip coil 13 jof the circuit lbreaker 2.V Until tired or 4.activated Vby'afsrnallV gate current in its gate electrode s,1the controlled rectifier-:8S blocks current flow in both directions and hence is effect an open circuit. When activated, however, it will abruptly change to a low-forward-impedance state which enables suicient current to ow in the tripping circuit lll-12413 of the circuit breaker 2 to effect actuation of the breaker latch 14. Since its anode current then exceeds a predetermined minimum value (the holding current) required to sustain conduction in a controlled rectifier of the type illustrated, this device will remain active until the breaker auxiliary switch l2 opens, even if the gate signal were quickly removed.

In order to initiate tiring of the controlled rectifier 35 whenever input terminal 82a is energized by a negativegoing control signal, a PNP transistor 86 and a pulse transformer 87 have been provided. As can be seen in FIG. 2, the transistor emitter is connected directly to the positive supply voltage terminal, while its collector is connected by way of a resistor 88 and the primary winding 87a of the pulse transformer 87 to the negative supply voltage terminal. The secondary winding 8717 of the pulse transformer is connected between the cathode and gate electro-de of the controlled rectifier 85. A diode 89 disposed across the primary winding 87a and poled as shown serves to limit the peak secondary voltage which can be induced in the winding 87b, upon deactivation of the static switch, to less than the maximum permissible reverse gate voltage of the controlled rectier 85. A connection is made from the base electrode of the transistor S6 to the junction between resistors 83 and 84, the resistor S4 being the base resistor of this transistor.

Normally the potential level at the input terminal 82a of the static switch 19 is nearly the same as that of the positive supply voltage terminal, negligible current can flow through the resistors 84 and 83, and the transistor S6 is necessarily turned olf (inactive). However, as soon as the output control signal is developed by the relay means 3a, the input terminal 82a will become energized by a negative potential almost equal to the magnitude of the supply voltage, and current flow is immediately effected in the emitter-base junction of the transistor S6. This activates the transistor 86 and causes a rapid current -increase in the primary winding 87a of the pulse transformer 87. As a result, the secondary winding 87h, which is connected in the gate circuit of the controlled rectier 85, supplies gate current in the proper direction and of appropriate magnitude and duration to re this device.

Since the controlled rectifier 85 is relatively sensitive, and only a small gate signal is required to fire it, it is important to prevent stray voltage transients or surges in the relay circuits from activating the static switch at some inpropitious moment. Toward this end, the gate circuit of the controlledV rectier is provided with a surge suppressing combination of a capacitor 9i)` of small capacitance, connected between cathode and gate electrode, and a very small resistor 91 serially connected between the gate electrode and the pulse transformer secondary winding 37b. In addition, as can be seen in FIG. 2, the anode-cathode circuit of the controlled rectier is provided with the surge suppressing series combination of an even smaller resistor 92 and a capacitor 93 connected thereacross. It will be recognized by those skilled in the art that these surge suppressing arrangements will absorb short-term transient surges thereby ensuring that the controlled rectifier will not be tired except, as described above, in response to a genuine control signal being applied to the input terminal 82a.

From the foregoing detail description of the circuitry and operation of the various functional components of the relay which is depicted in FIG. 2, the overall mode of operation may now be readily followed. For this purpose it will iirst be assumed that the protected circuit 1a has been subjected to an abnormality which causes a sudden increase in circuit current to an overcurrent value i 21/2 times greater than pickup. This is a relatively mild liti overcurrent condition, and the circuit current is still within the aforementioned first range of overcurrent values. Therefore the saturable transforming means 31 is operating in its linear region, and the value of secondary A.C. voltage derived thereby will be 21/2 times the 4-volt pickup level, or l0 volts R.M.S. Of course the starting means 60 instantaneously responds to any transformer secondary voltage in excess of pickup and it activates the transistor 59 in the shunt circuit means 58 of the supervising means 49. As a result, the normally conducting diode 55, which had been preventing the timing capacitor 47 of the energy storing means 25 from charging, has positive supply voltage potential applied to its negative pole and is thereby rendered non-conductive, whereupon the capacitor 47 immediately begins accumulating energy supplied thereto by its D.C. energizing circuit.

The timing capacitor 47 is supplied with a high-frequency succession of triangular energizing pulses. This is illustrated in FIG. 6 which is a voltage-time chart, the interval of time shown corresponding to the duration of just a half-cycle of circuit current. In FIG. 6 the heavy-line triangles identified by the reference numeral 94 depict net voltage appearing between conductors A and D in the relay circuit of FIG. 2, which voltage is applied to the RC time delay circuit including the potentiometers 45 and 46 and the capacitor 47. These voltage triangles or pulses 94 recur at a constant frequency of 2,000 c.p.s., as determined by the sawtooth generator 23 which is producing, between conductors B and C in the adding circuit 22 of FIG. 2, a triangular waveform A.C. voltage (reference numeral 95 in FIG. 6) of constant amplitude Vs. The height and duration of each net voltage pulse 94 is determined by the D.C. voltage (the broken-line IBO-degree sine wave 96 in FIG. 6) which the rectilier 33 provides between conductors A and B in FIG. 2. The D.C. voltage 96, being derived from the protected circuit 1a by way of transformer 31 and hence being representative of the circuit current, has a magnitude of 10 volts R.M.S., corresponding to 21/2 times pickup. Since the net voltage 94 comprises the sum of these A.C. and D.C. voltages 95 and 96 less the unipolarity reference voltage level VZ which is introduced by the impedance means 41 between conductors C and D, with VZ being equal to Vs, the height of each net voltage triangle is in fact equal to the instantaneous magnitude of the D.C. voltage 96 at the same moment of time, and it follows that the integrated area of the energizing pulses 94 during a full cycle of circuit current is directly proportional to the square of the eifective magnitude of the D.C. voltage 96.

The timing capacitor 47 is incrementally charged by the chain of triangular energizing pulses 94 until the predetermined critical energy level is attained therein. In theexample being considered it is assumed that the potentiometers 45 and 46 have been adjusted so that this will take about nine-tenths of a second, which is more than one-hundred times longer than the time interval shown in FIG. 6. Since the capacitor voltage at its critical level is only two Volts, it is apparent that at 21/2 times pickup the average voltage gain per energizing pulse is very small. At the expiration of the designated time delay, the capacitor charge inally attains the predetermined magnitude which, in conjunction with the 8volt bias provided by the resistor S6 in circuit therewith, will raise the emitter voltage on the unijunction transistor 67 of the level detector 26 to its critical l0-Volt level. This is the peak or ring point of the unijunction transistor 67 whenever the sampling means 70 is active. v

The sampling means 70, which is periodically active at the rate of 300 times a second, will be active and thereby enable the unijunction transistor 67 to fire at some time within .GOB-second following the critical energy level attainment in capacitor 47. In response to this event, the signal amplifying transistor 7S is turned on 17 and a negative-going output control signal is produced at 18a. In the illustrated embodiment of the invention, this output signal initiates opening of a circuit breaker by activating the transistor 86 which causes a firing signal tobe developed in the gate circuit of the controlled rectifier 85 of the static switch 19. As is indicated in FIG. 2, the consequent firing or activation of the controlled rectifier completes the energizing circuit for the breaker trip coil 13. The operating example just considered has been represented in FIG. 3 by a point 97 shown on curve 30, which curve depicts the operating characteristic obtained for the illustrated relay with the assumed timing adjustment.

Reference has been made hercinbefore to the desirable departure of the operating characteristic curve 30 from a true Izt relationship at its relatively low-overcurrent end, i.e.,y to the left of point 97 as viewed in FIG. 3. This is an 'inherent characteristic of the disclosed relay, and it will best be understood by examining FIG. 6. In FIG. 6 it is apparent that the height of the first and last energizing pulses 94 during each half-cycle of circuit current is quite small. As the voltage of the timing capacitor 47 approaches its critical 2-volt level near the end of the designated time delay, it will surpass the magnitude of at least one of these two pulses. Consequently, some vof the periodic energizing pulses will not be contributing anything to the charge accumulating process, and the relay operating time will be slightly extended. This effect is even more significant during lower-overcurrent, longeroperating-time conditions. For example, at 11/2 times pickup there are around five or six energizing pulses every cycle of circuit current Whose magnitudes are less than two volts, and the cumulative loss of their charging contributions as the capacitor voltage very slowly approaches its critical level, more than two seconds after the timing operation began, will be reflected in an appreciahly longer time delay than would be obtained with a true I2t relationship.

In further explanation of the mode of operation of the illustrated inVerse-thne-overcurrent relay, a second operating example will now be considered. Let it be assumed that a more severe abnormal circuit condition has occurred, and the current in circuit 1a suddenly rises to an over-current value 8 times greater than pickup. This current value, as will be explained hereinafter, is beyond the aforementioned first range of overcurrent values, and consequently the saturable transforming means 31 will be in its non-linear region. Assume, therefore, that the secondary A.-C. voltage derived by the transforming means 31 is only 7?/2 times the 4-volt pickup level, or 30 volts R.M.S.

. The starting means 60 will again respond instantaneously to activate the shunt circuit means 53, whereupon the supervising means 49 enables the timing capacitor 47 immediately to start accumulating energy. Since the D.C. voltage which is applied to the adding circuit 22 by rectifier 33 reflects the increased value of secondary voltage derived by the transformer 31, the timing circuit 25 is now energized by a train of triangular net voltage pulses having three times the height and duration of those shown at 94 in FIG. 6. But three times the duration (base) of those several triangular pulses of highest magnitude (the pulses which happen to fall between 70 and 110 electrical degrees on the lSO-degree time interval shown in FIG. 6) will exceed the period of the high-frequency lA.'-C. voltage which is supplied by the sawtooth generator 23, and consequently the energization supplied to the timing circuit, for the particular condition assumed, will cease being a periodic` quantity and will actually become continuous for approximately 40 electrical degrees every half cycle of circuit current. It is apparent, therefore, that the integrated area of the energizing pulses during'a full cycle of circuit current at 8 times pickup will be slightly less than ninetimesjgreater than the corresponding area at 2,1/2 times pickup.

Under the more severe overcurrent condition now being considered, with no change in the relays timing adjustment, the capacitor 47 will take about ten times longer than the time interval shown in FIG. 6 to accumulate the predetermined amount of energy required for firing the unijunction transistor 67 when the sampling means 70 is active. As before, this response of the level detector 26 produces an output control signal at 18a, andthe static switch 19 is activated thereby to perform its breaker trip coil energizing function,

From the foregoing description of operation at 8 times pickup, it will be apparent that when the maximum magnitude of the D.C. voltage applied to the adding circuit 22 of the relay exceeds the peak-to-peak magnitude (40 volts) of the pulsating triangular signal, the value of the net D.C. signal energizing the time delay circuit 25 will be continuously finite for at least two successive cycles of the sawtooth generator 23 each half cycle of circuit current, and the relay tends to operate more slowly than would be expected at high multiples of pickup with a true Izt relationship. This tendency, which becomes more pronounced as the circuit current increases, is augmented by the nonlinearity of transforming means 31 which tends to limit the D.-C. voltage, relative to the circuit current from which it is derived, whereby this voltage increases proportionately less than circuit current.

In practice I use transforming means designed to begin saturating when the current in circuit 1a attains a value between four and eight times pickup, preferably, about six times. Up to this point, which defines the aforementioned rst range of overcurrent values, the transforming means has performed in a linear fashion, but thereafter it progressively saturates and hence is increasingly effective in limiting the magnitude of the representative D.-C. voltage which is derived from the protected circuit. By arranging the transforming means 31 to have an appropriately high degree of nonlinearity, I am able to attain the desired deviation of the relay operating characteristic 3ft (FIG. 3), in a prolonged-time sense, from a true I2! relationship at its relatively high-overcurrent end.

In order to illustrate the manner in which the desired non-linear characteristic of the transforming means 31 might be obtained, I have shown in FIG. 7 a simplified physical version of such a device. As it is represented in FIG. 7, the saturable transforming means 31 comprises a current energized primary winding 98 and an A.-C. voltage deriving secondary Winding 99 which are disposed, respectively, on the companion legs 100a and 100b of a common, magnetizable metalv core member 100. The core is not endless but has an air gap 101 therein.' Those skilled in the art will recognize that the primary current value at which this device will initially enter its saturated or nonlinear region, and the degree of nonlinearity thereafter, are determined by appropriately `controlling the following design factors: the number of turns in the primary winding 98; the material of the metal core 10i); the cross-sectional area of the core; and the length of the air gap 101. Y

While I have shown and described a preferred form of the invention by way of illustration, many modifications will occur to those'skilled in the art. It is contemplated, therefore, by the claims which conclude this specification to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by'United States Letters Patent is:

,1. Relay means Vfor initiating a predetermined control function in delayed response to the occurrence, in an alternating current circuit, of an' overcurrent condition as indicated by the circuit vcurrent, increasing beyond a predetermined pickup value, the relay means having an inverse-time-overcurrent operating characteristic, comprising: saturable transforming means includingprimary and secondary windings disposed on a magnetizable core havingV an air gap, said primary winding being coupled to the4 circuit and said secondary winding having induced therein an A.C. voltage which is linearly representative of the alternating current in the circuit for a first range of overcurrent values, said first range including values up to at least four times but not more than eight times said predetermined pickup value, said transforming means being designed to begin saturating, and to become progressively more saturated, for increasing overcurrent values exceeding those of said first range; rectifying means connected to said secondary Winding for providing a D.-C. signal having a magnitude determined by the value of said A.C. voltage; a signal source for supplying a triangular Waveform A.C. signal of substantially constant amplitude; adding means connected to said rectifying means and to said signal source for providing a resultant signal corresponding to the sum of the D,-C. and A.-C. signals; means associated with said adding means for introducing, in subtractive relationship with the resultant signal, a unipolarity reference signal level equal to the amplitude of said A.C. signal; electric energy storing means, including a reactance element, connected to said adding means for energization in accordance with the difference between the resultant signal magnitude and the reference signal level whenever the former exceeds the latter, whereby the time required for said reactance element to accumulate a predetermined amount of energy following the occurrence of an overcurrent condition is inversely related to the value of the alternating current in the circuit; and level detecting means, connected to said reactance element, adapted to initiate the predetermined control function in response to the element accumulating said predetermined amount of energy.

2. Means for initiating a predetermined control function in delayed response to the occurrence of an abnormal condition in an electric current circuit, the amount of delay being inversely related to the severity of the abnormal condition, comprising: condition responsive means adapted to be coupled to the circuit for deriving therefrom a D.-C. signal having a magnitude which is dependent upon the value of a characteristic electric quantity of the circuit; a pulsating signal source for supplying a regular succession of triangular-Waveform signal pulses; adding means connected to said condition responsive means and to said pulsating signal source for providing a resultant signal corresponding to the sum of the D.C. and pulsating signals; impedance means associated with said adding means for introducing, in subtractive relationship with the resultant signal, a unipolarity reference signal level equivalent to the peak magnitude of said signal pulses; electric energy storing means, including a reactance element, connected to said adding means for energization in accordance with the difference between the resultant signal magnitude and the reference signal level whenever the former exceeds the latter, whereby the time required for the energy being accumulated in said reactance element to build up to a predetermined critical level following the occurrence of an abnormal circuit condition is inversely related to the value of said characteristie electric quantity; and level detecting means, connected to said reactance element, adapted to initiate the predetermined control function in response to the attainment in said element of an energy level which is greater than said predetermined critical level, said level detecting means having associated therewith periodically active sampling means for effecting a suiiicient increase in the sensitivity of the level detecting means to enable the level detecting means to respond, at predetermined time intervals, to the accumulation in said reactance element of said predetermined critical level of energy.

3. The relay means of claim 2 in which each of the predetermined time intervals at which the sampling means elects said increased Sensitivity of the level detecting means is from two to eight times longer than the period of each triangular-waveform signal pulse. i 4. vIn an overcurrent protective relay for an alternatmg current circuit, the combination of condition responsive means adapted to be coupled to the circuit for deriving therefrom a representative D.C. signal; a pulsating signal source for supplying a rapid succession of triangular-Waveform signal pulses; impedance means for establishing a unipolarity reference signal level equivalent to the peak magnitude of the pulsating signal; energy storing means, including a capacitor, disposed for energization by a quantity derived from the D.C. signal plus the pulsating signal minus the reference signal; level detecting means, comprising a unijunction transistor the base-one-emitter circuit of which is connected to said capacitor, for producing an output control signal upon activation of the unijunction transistor, the level detecting means being so arranged that the unijunction transistor nominally is activated in response to the voltage across said capacitor attaining a first predetermined level of magnitude; and periodically active sampling means associated with said level detecting means for effecting, at recurrent intervals of predetermined duration, a suiicient increase in the sensitivity of the level detecting means to enable activation of the unijunction transistor to take place in response to the attainment of a critical level of capacitor voltage which is lower than said first level.

5. In an inverse-time overcurrcnt protective device for an alternating current circuit, the combination of: means adapted to be coupled to the alternating current circuit for deriving therefrom a D.C. signal having a magnitude which is dependent upon the value of current in the circuit; electric energy storing means, including a capacitor, disposed for energization by an electric quantity which varies in accordance with the magnitude of said D.-C. signal; level detecting means, comprising a unijunction transistor the base-one-emitter circuit of which is connected to said capacitor, for producing an output control signal upon activation of the unijunction transistor, the level detecting means being so arranged that the unijunction transistor nominally is activated in response to the capacitor being charged a first predetermined amount; and periodically active sampling means associated with the level detecting means for effecting, at recurrent intervals of predetermined short duration, a sufficient increase in the sensitivity of the level detecting means to enable the uniiunction transistor to be activated in response to the capacitor being charged a critical amount which is less than said first predetermined amount.

6. Relay means for initiating a predetermined control function in delayed response to the occurrence, in an alternating current circuit, of an overcurrent condition as indicated by the circuit current increasing beyond a predetermined pickup value, comprising: saturable transforming means adapted to be coupled to the circuit for deriving therefrom an A.C. voltage which is linearly representative of the alternating current in the circuit for a first range of overcurrent values, said yfirst range including values up to at least four times but not more than eight times said predetermined pickup value, and said transforming means being designed to begin saturating, and to become progressively more saturated, for increasing overcurrent values exceeding those of said first range; rectifying means connected to said saturable transforming means for providing a D.C. signal having a magnitude determined by the value of said A.C. voltage; electric energy storing means, including a capacitor, coupled to said rectifying means, the energization of said energy storing means being controlled by said D,-C. signal whereby the time required for said capacitor to charge to a predetermined critical level of voltage following the occurrence of an overcurrent condition is inversely related to the amount of overcurrent; level detecting means, comprising a unijunction transistor the base-one-emitter circuit of which is connected to said capacitor, adapted to initiate the predetermined control function upon activation of the unijunction transistor, the level detecting means being so arranged that the unijunction transistor nominally is activated in response to said capacitor being charged to a voltage level ywhich is higher than said predetermined critical level; and periodically act-ive sampling means associated 'with the level detecting means for eiecting a suicient increase in the sensitivity of the level detecting means vfor effecting a suiiicient increase in the sensitivity of the level detecting means to enable the unijunction transistor to be activated, at predetermined time intervals, in response to said capacitor being charged to said predetermined critical voltage level.

7. In an inverse-time overcurrent protective device for an alternating current circuit, the combination of: means adapted to be coupled to the alternatinig current circuit for deriving therefrom a signal dependent upon the value of circuit current; energy storing means including a resistor and a capacitor connected in series combination; means controlled by said signal for providing a succession of D.C. energizing pulses for said energy storing means, thereby incrementally charging said capacitor; and level detecting means, connected to said capacitor, adapted to initiate a predetermined control function in response to the capacitor being charged a predetermined amount, said level detecting means having associated therewith periodically active sampling means for effecting a sufficient increase in the sensitivity of the level detecting means to enable the level detecting means to respond, whenever the sampling means is active, to the capacitor being charged a critical amount which is less than said predetermined amount.

8. In an inverse-time overcurrent protective device, the combination of: means adapted to be coupled to a source of alternating current for providing a succession of D.C. energizing pulses dependent upon the value of said a1- ternating current; energy storing means comprising a capacitor connected to said first-mentioned means, Whereby said capacitor is incrementally charged by said energizing pulses; and level detecting means, connected to said capacitor, adapted to initiate a predetermined control function in response to the capacitor being charged a predetermined amount, said level detecting means having associated therewith periodically Iactive sampling means for eiiecting a sufficient increase in the sensitivity of the level detecting means to enable the level detecting means to respond, at predetermined time intervals, to the capacitor being charged a critical amount which is less than said predetermined amount.

9. ln an lovercurrent protective device, the combination of: means adapted to be coupled to a source of alternating current for producing a pulsating D.C. energizing signal dependent upon the value of said alternating current; means comprising a capacitor connected to said trst-mentioned means for energization in accordance with said signal; level detecting means, comprising a unijunction transistor the baseoneemitter circuit of which is connected to said capacitor, for producing an output control signal upon activation of the unijunction transistor, the level detecting means being so arranged that the unijunction transistor nominally is activated in response to the capacitor being charged a `iirst predetermined amount; and periodically active sampling means connected to the unijunction transistor for effecting at frequently rewrring intervals a momentary increase in the sensitivity of the level detecting means to enable the unijunction transistor to be activated in response to the capacitor being charged a critical amount which is less than said tirst predetermined amount.

References Cited by the Examiner UNITED STATES PATENTS 2,735,962 2/56 Ellis 317-27 X 2,977,510 3/61 Adamson 317-36 3,679,533 2/63 Kotheimer 317-36 SAMUEL BERNSTEIN, Primary Examiner.

LLOYD MCCOLLUM, Examiner. 

2. MEANS FOR INITIATING A PREDETERMINED CONTROL FUNCTION IN DELAYED RESPONSE TO THE OCCURRENCE OF AN ABNORMAL CONDITION IN AN ELECTRIC CURRENT CIRCUIT, THE AMOUNT OF DELAY BEING INVERSELY RELATED TO THE SEVERITY OF THE ABNORMAL CONDITION, COMPRISING: CONDITION RESPONSIVE MEANS ADAPTED TO BE COUPLED TO THE CIRCUIT FOR DERIVING THEREFROM A D.C. SIGNAL HAVING A MAGNITUDE WHICH IS DEPENDENT UPON THE VALUE OF A CHARACTERISTIC ELECTRIC QUANTITY OF THE CIRCUIT; A PULSATING SIGNAL SOURCE FOR SUPPLYING A REGULAR SUCCESSION OF TRIANGULAR-WAVEFORM SIGNAL PULSES; ADDING MEANS CONNECTED TO SAID CONDITION RESPONSIVE MEANS AND TO SAID PULSATING SIGNAL SOURCE FOR PROVIDING A RESULTANT SIGNAL CORRESPONDING TO THE SUM OF THE D.C. AND PULSATING SIGNALS; IMPEDANCE MEANS ASSOCIATED WITH SAID ADDING MEANS FOR INTRODUCING, IN SUBTRACTIVE RELATIONSHIP WITH THE RESULTANT SIGNAL, A UNIPOLARITY REFERENCE SIGNAL LEVEL EQUIVALENT TO THE PEAK MAGNITUDE OF SAID SIGNAL PULSES; ELECTRIC ENERGY STORING MEAN, INCLUDING A REACTANCE ELEMENT, CONNECTED TO SAID ADDITING MEANS FOR ENERGIZATION IN ACCORDANCE WITH THE DIFFERENCE BETWEEN THE RESULTANT SIGNAL MAGNITUDE AND THE REFERENCE SIGNAL LEVEL WHENEVER THE FORMER EXCEEDS THE LATTER, WHEREBY THE TIME REQUIRED FOR THE ENERGY BEING ACCUMULATED IN SAID REACTANCE ELEMENT TO BUILD UP TO A PREDETERMINED CRITICAL LEVEL FOLLOWING THE OCCURRENCE OF AN ABNORMAL CIRCUIT CONDITION IS INVERSELY RELATED TO THE VALUE OF SAID CHARACTERISTIC ELECTRIC QUANTITY; AND LEVEL DETECTING MEANS, CONNECTED TO SAID REACTANCE ELEMENT, ADAPTED TO INITIATE THE PREDETERMINED CONTROL FUNCTION IN RESPONSE TO THE ATTAINMENT IN SAID ELEMENT OF AN ENERGY LEVEL WHICH IS GREATER THAN SAID PREDETERMINED CRITICAL LEVEL, SAID LEVEL DETECTING MEANS HAVING ASSOCIATED THEREWITH PERIODICALLY ACTIVE SAMPLING MEANS FOR EFFECTING A SUFFICIENT INCREASE IN THE SENSITIVITY OF THE LEVEL DETECTING MEANS TO ENABLE THE LEVEL DETECTING MEANS TO RESPOND, AT PREDETERMINED TIME INTERVALS, TO THE ACCUMMULATION IN SAID REACTANCE ELEMENT OF SAID PREDETERMINED CRITICAL LEVEL OF ENERGY. 